IntroductionWhen dealing with
the cursor in most high-level languages, the cursor is defined as the place
where the next text output will appear on the display. When dealing directly
with the display, the cursor is simply a blinking area of a particular
character cell. A program may write text directly to the display independent
of the current location of the cursor. The VGA provides facilities for
specifying whether a cursor is to be displayed, where the cursor is to
appear, and the shape of the cursor itself. Note that this cursor is only
used in the text modes of the standard VGA and is not to be confused with
the graphics cursor capabilities of particular SVGA chipsets.

Enabling/Disabling the Cursor On the VGA there are three
main ways of disabling the cursor. The most straightforward is to set the
Cursor Disable field to 1. Another way is
to set the Cursor Scan Line End field to a
value less than that of the Cursor Scan Line Start
field. On some adapters such as the IBM EGA, this will result instead in
a split block cursor. The third way is to set the cursor location to a
location off-screen. The first two methods are specific to VGA and compatible
adapters and are not guaranteed to work on non-VGA adapters, while the
third method should.

Manipulating the Cursor PositionWhen dealing with
the cursor in standard BIOS text modes, the cursor position is specified
by row and column. The VGA hardware, due to its flexibility to display
any different text modes, specifies cursor position as a 16-bit address.
The upper byte of this address is specified by the Cursor
Location High Register, and the lower by the Cursor
Location Low Register. In addition this value is affected by the Cursor
Skew field. When the hardware fetches a character from display memory
it compares the address of the character fetched to that of the cursor
location added to the Cursor Skew field. If
they are equal and the cursor is enabled, then the character is written
with the current cursor pattern superimposed. Note that the address compared
to the cursor location is the address in display memory, not the address
in host memory. Characters and their attributes are stored at the same
address in display memory in different planes, and it is the odd/even addressing
mode usually used in text modes that makes the interleaved character/attribute
pairs in host memory possible. Note that it is possible to set the cursor
location to an address not displayed, effectively disabling the cursor.
The Cursor
Skew field was used on the EGA to synchronize the cursor with internal
timing. On the VGA this is not necessary, and setting this field to any
value other than 0 may result in undesired results. For example, on one
particular card, setting the cursor position to the rightmost column and
setting the skew to 1 made the cursor disappear entirely. On the same card,
setting the cursor position to the leftmost column and setting the skew
to 1 made an additional cursor appear above and to the left of the correct
cursor. At any other position, setting the skew to 1 simply moved the cursor
right one position. Other than these undesired effects, there is no function
that this register can provide that could not be obtained by simply increasing
the cursor location.

Manipulating the Cursor Shape On the VGA, the text-mode
cursor consists of a line or block of lines that extend horizontally across
the entire scan line of a character cell. The first, topmost line is specified
by the Cursor Scan Line Start field. The last,
bottom most line is specified by the Cursor Scan
Line End field. The scan lines in a character cell are numbered from
0 up to the value of the Maximum Scan Line
field. On the VGA if the Cursor Scan Line End
field is less than the Cursor Scan Line Start
field, no cursor will be displayed. Some adapters, such as the IBM EGA
may display a split-block cursor instead.

Cursor Blink Rate On the standard VGA, the
blink rate is dependent on the vertical frame rate. The on/off state of
the cursor changes every 16 vertical frames, which amounts to 1.875 blinks
per second at 60 vertical frames per second. The cursor blink rate is thus
fixed and cannot be software controlled on the standard VGA. Some SVGA
chipsets provide non-standard means for changing the blink rate of the
text-mode cursor.

Cursor Color On the standard VGA, the
cursor color is obtained from the foreground color of the character that
the cursor is superimposing. On the standard VGA there is no way to modify
this behavior.